The studies proposed are designed to purify and characterize colony stimulating factors from human sources with the following objectives in mind: 1. The biochemical characterization of the purified factors. 2. The preparation of specific antibodies and the use of these antibodies for the development of radioimmunoassays and the possible use as a probe to identify and purify the committed macrophage-granulocyte progenitor cell. 3. Elucidate the mechanisms of action of CSF and its role in normal and abnormal granulopoiesis.